1. Field of the Invention
The present invention relates to an X-ray source for radiating X-rays with low energy at a microscopic focus.
2. Description of the Related Art
A general X-ray source having a microscopic focus has already been produced as a micro-focus X-ray source, and is widely used in equipment such as a nondestructive inspection apparatus for inspecting a microscopic area of a target at a high resolution. This X-ray source employs a configuration in which electron beams radiated from an electron source are converged by means of an electro-optical system such as an electric field or magnetic field lens, a focus is provided in a small area that is equal to or smaller than the order of micrometers on a surface of a transmission target, and the X-rays are radiated at the focus while the transmission target is transmitted (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-28845 (pages 4 to 5 and FIG. 1)).
It is important to make a design while obtaining matching between an electron source and an electro-optical system in order to converge electron beams at a small spot with a constant amount of current, whereas an X-ray source having a microscopic focus close to 0.1 μm is achieved by making a variety of contrivances at the present stage.
An X-ray source that carries out transmission exposure of an inspection target at a high resolution is required to have the microscopic focus as described above in achieving a spatial resolution, whereas it becomes important to enable radiation of X-rays with energy suitable to achieve high contrast. This is because, when transmission exposure of an inspection site in a microscopic area is carried out, if the energy of X-rays to be used is too high, the contrast of an exposure image cannot be obtained, making it impossible to judge the presence or absence of a defect.
Most of the current micro-focus X-ray sources are driven at a high voltage equal to or greater than 70 or 150 kV, and radiate X-rays with high energy. However, in the case where an inspection target is a small sample having size of several tens of μm, or a constituent element thereof is a light element with a small X-ray damping rate, in particular, an organic substance, it becomes necessary to use X-rays with low energy covering a soft X-ray area, which is equal to or smaller than 30 keV, or occasionally, equal to smaller than 5 keV. Further, in recent years, there has been a growing demand for high resolution inspection in the field of products in which organic materials are frequently used, in the field of pharmaceutics and relative to microscopic targets composed of light elements such as cells. Therefore, a need exists for practical use of an X-ray source having a microscopic focus, which is capable of radiating X-rays with low energy covering the soft X-ray area described above.
However, in the case where it becomes possible to radiate X-rays with low energy with a conventional configuration of a micro-focus X-ray source kept unchanged, this caused the following problems to be solved.
The physical constraints that occur when an attempt is made to converge electron beams with low energy in a small area primarily include two problems described below. One problem is that a divergence action occurs due to a spatial electric charge effect at the time of crossover of electron beams in an electro-optical system. The other problem is that, with lower energy, the blurring quantity of a focus on an image forming face increases under the strong influence of the magnetic field of the electro-optical system or color aberration of an electric field lens.
The physical constraints in achieving radiation intensity (dosage) of X-rays with low energy primarily include two problems described below. One of these problems is that it is disadvantageous to apply electron beams with low energy from the viewpoint of increase in dosage because the radiation quantity of controlled X-rays is substantially proportional to energy of excitation electrons. The other problem is that it becomes difficult to achieve radiation intensity of X-rays with low energy due to the attenuation (absorption) effect associated with transparent target.
Therefore, it is impossible to maintain the initial microscopic focus size merely by reducing and operating a drive voltage of a current micro-focus X-ray source with high energy. In addition, it is difficult to include the limit of an achievable focus size in a satisfactory range merely by making a design change so as to cope with low voltage driving with a configuration thereof kept unchanged. For this reason, a contrivance on the configuration of the X-ray source is required to achieve microscopic focus performance, which is capable of reducing energy and radiating soft X-rays with sufficient intensity (dosage) at high efficiency, and which is equal to or more excellent than that of the current micro-focus X-ray source with high energy.